Titanium is a light metal having a high mechanical strength to weight ratio and exhibiting superior corrosion resistance. Titanium is widely used in various fields including airplane, medical and automobile industries. A quantity of consumed titanium use has been increasing. Titanium is the fourth most abundant element in the earth's crust after aluminum, iron, and magnesium among metal elements and thus is a plentiful resource. Although titanium is a plentiful resource, titanium is up against short supply and has been at least an order of magnitude more expensive than steel materials.
Titanium metal has been mainly produced by a Kroll Process. In the Kroll Process, titanium ore containing titanium dioxide (TiO2) as a main component is reacted with a chlorine gas and coke (C) to produce titanium tetrachloride (TiCl4). Subsequently, highly-purified titanium tetrachloride is produced through distillation and separation. Titanium metal is produced through thermal reduction of the purified titanium tetrachloride with magnesium (Mg). In the thermal reduction step of the Kroll Process, a reduction reaction vessel made of a stainless steel is filled with a magnesium melt at a temperature of not lower than 800° C. Titanium tetrachloride in a liquid phase is dropped into the vessel from above and reacts with the magnesium melt in the vessel to produce titanium. The produced titanium sinks in the magnesium melt and thus titanium is produced in a sponge form. By-product magnesium chloride and unreacted magnesium in the liquid phase are mixed in the titanium in the sponge form. Upon completion of the reaction, the reaction mixture is subjected to a vacuum separation process at a high temperature of not lower than 1000° C. to obtain a sponge cake of porous titanium. The sponge cake is cut and crushed to produce sponge titanium.
The Kroll Process can effectively produce a titanium material for practical use. However, a long production time is required since the thermal reduction process and the separation process are conducted separately. The production is less efficient since it is a batch process. Accordingly, various techniques have been suggested to overcome the problems of the Kroll Process.
For example, Patent Literature 1 (JP-B1-33-3004) discloses a process including steps of supplying a titanium tetrachloride gas and magnesium vapor in a reaction vessel to cause a gas-phase reaction at a temperature of 800 to 1100° C. and under a vacuumed atmosphere of 10−4 mmHg (1.3×10−2 Pa) in the vessel, and depositing titanium on a net-like collection material disposed in the vessel to collect titanium.
Patent Literature 2 (U.S. Pat. No. 2,997,385) discloses a process for producing a metal, including steps of introducing halide vapor as a metal element and alkali metal or alkaline earth metal vapor as a reducing agent into a reaction vessel, and causing a gas-phase reaction in the vessel in an evacuated atmosphere under a pressure of 0.01 to 300 mmHg (1.3 Pa to 40 kPa) and at a temperature of 750 to 1200° C. Patent Literature 2 discloses, in Example II, a method for producing titanium from TiCl4 gas and Mg gas, and specifically, the reaction was caused at a reaction temperature of approximately 850° C. and under a pressure of 10 to 200 microns (1.3 to 26.7 Pa).
In Patent Literature 3, titanium particles are supplied in a reaction vessel while titanium tetrachloride gas and magnesium gas are separately injected into the vessel. The titanium particles are floated by the energy of the injection, and a reduction reaction of titanium tetrachloride by magnesium is produced to adhere and accumulate the reduced titanium metal on surfaces of the titanium particles. Patent Literatures 4 and 5 disclose to use a fluidized bed for reducing titanium tetrachloride by magnesium and for depositing so that small titanium particles are produced by the reaction. In Patent Literature 4, produced titanium particles are recycled and further deposited to become larger particles.